Abstract: :
Purpose: To date no systematic method has been used for characterisingthe residual visual capacity of blindsight subjects that wouldallow comparison and generalisation across all subjects. Weaimed to provide such comparison and apply it to a large numberof cortically blind patients to address both the question ofprevalence and characteristics of blindsight. Methods:Detectionof suprathreshold gratings in a two alternative forced-choiceparadigm provides a criterion free measure of spatial processingwithin the field defect. The presence of a band-pass spatialchannel has previously been reported in one blindsight subject.We have applied the same methodology to study eight corticallyblind subjects with V1 lesions. Magnetic Resonance Imaging wasused to determine the extent of brain damage in all cases. Results:We have demonstrated the presence of blindsight in seven outof eight subjects tested. The eighth subject had a relativelarge lesion extending outside of V1. The spatial channels mediatingblindsight appear to have a band-pass frequency response characteristicsin two subjects and low-pass in five subjects. The higher cut-offfrequencies in all cases were below 4 cycle/degree. The lowercut-off frequency of the band-pass channel was approximately0.5 cycle/degree. Conclusion: Blindsight in cortically blindsubjects may not be a rare phenomenon. Our study also indicatesthat the spatial channels giving rise to blindsight performanceare tuned to low spatial frequencies, consistent with mediationby subcortical processes.